Kojo wants to be an author. To improve his writing skills, every day after school he writes $3$ pages of a story he is working on. Kojo has been writing his story for $d$ days. He has a total of $81$ pages completed. Write an equation to describe this situation. How many days has Kojo been writing his story?
Solution: Kojo writes ${3}$ pages each day. He has been writing for an unknown number of days, which we're calling ${d}$. He has written a total of ${81}$ pages. We can represent the total number of pages that Kojo has written as a product: ${3} {d}$ We know that he has written ${81}$ pages. We can set these two expressions equal to describe this situation with an equation: ${3} {d} = {81}$ Other ways to represent the situation with an equation include: $\dfrac{{81}}{ d}=3$ or $\dfrac{{81}}{ 3}= d$. Now we can solve for ${d}$. Divide both sides by ${3}$ to get $ d$ by itself. $\begin{aligned}\dfrac{ {3}{d}}{3} &= \dfrac{{81}}{3} \\\\ {d} &={ 27} \end{aligned}$ The following equation matches this situation: $3d=81$ Kojo has been writing his story for $27$ days.